What is the process of transcription. Transcription in biology - what is it? Definition of "transcription in biology"

TRANSCRIPTION in biology(syn. template RNA synthesis) - synthesis of ribonucleic acid on a matrix of deoxyribonucleic acid. T., which occurs in living cells, is First stage implementation of genetic traits contained in DNA (see Deoxyribonucleic acids). As a result of T., RNA is formed (see Ribonucleic acids) - an exact copy of one of the DNA strands according to the sequence of nitrogenous bases in the polynucleotide chain. T. is catalyzed by DNA-dependent RNA polymerases (see Polymerases) and provides the synthesis of three types of RNA: messenger RNA (mRNA) encoding the primary structure of the protein, that is, the sequence of amino acid residues in the building iolipeptide chain (see Proteins, biosynthesis); ribosomal RNA (rRNA) that make up ribosomes (see), and transport RNA (tRNA) involved in the process of protein synthesis as a component that "recodes" the information contained in mRNA.

T. in microorganisms has been studied more fully than in higher organisms (see Bacteria, genetics). The process of T., catalyzed by RNA polymerase, is divided into 4 stages: the binding of RNA polymerase to DNA, the beginning - initiation - of the synthesis of an RNA chain, the actual process of synthesis of a polynucleotide chain - elongation, and the completion of this synthesis - termination.

RNA polymerase has the highest affinity for certain regions of the DNA template containing a specific nucleotide sequence (the so-called promoter regions). Enzyme binding to such a site is accompanied by partial local melting of DNA strands and their separation. At the stage of initiation, the inclusion of the first nucleotide - usually adenosine (A) or guanosine (G) - into the RNA molecule occurs. During elongation, RNA polymerase locally unwinds the DNA double helix and copies one of its strands according to the principle of complementarity (see Replication). As the RNA polymerase moves along the DNA, the growing RNA chain moves away from the template, and the double-stranded structure of the DNA is restored after passing through the enzyme. Termination of RNA synthesis also occurs at specific regions of DNA. In some cases, additional proteins are needed to recognize termination signals, one of which is the p-factor, which is a protein with ATPase activity, in other cases it may be modified nitrogenous bases. When the RNA polymerase reaches the terminator region, the synthesized RNA strand is finally separated from the DNA template.

The functional transcription unit in microorganisms is the operon (see), which includes one promoter, one operator and a number of genes encoding polypeptide chains (see Gene). T. operon begins with the stage of binding of RNA polymerase to the promoter - a site located at the very beginning of the operon. Immediately after the promoter is the operator - a DNA region that can bind to the repressor protein. If the operator is free, then T. of the entire operon occurs, but if the operator is associated with a repressor protein, T. is blocked. All well-studied repressors are proteins capable of undergoing allosteric changes (see Conformation). The structure of repressor proteins is encoded by regulatory genes located either immediately before the operon or at a considerable distance from it. The synthesis and activity of repressors are determined by the conditions of the extra- and intracellular environment (the concentration of metabolites, ions, etc.).

Transcription of DNA in higher organisms is carried out by separate sections called T. units - transcriptons. The structure of the T. unit includes the DNA of the corresponding gene and areas adjacent to it. Ideas about the structure of units of T. received significant development in connection with the identification of functional unequal sequence of eukaryotic gene regions. It turned out that within the structural genes of higher organisms are the so-called. introns are intercalated DNA sequences that are not directly related to the coding of a given protein. The number and size of introns of different genes vary greatly, in many cases the total length of all introns significantly exceeds the length of the coding part of the genes (exon). Clarification of a role of introns - one of actual tasks of molecular genetics (see).

In the process of T., RNA is formed, which is a copy of the entire transcription unit. In cases where genes code for protein synthesis, the primary product of T. is called the nuclear precursor of mRNA (pro-mRNA), and is several times larger than mRNA. Pro-mRNA includes sequences transcribed at coding regions (exons), introns, and, possibly, adjacent DNA regions. In the cell nucleus, pro-mRNA turns into mature mRNA, the so-called. processing, or maturation. At the same time, specific enzymes interact with pro-mRNA and selectively remove excess sequences, in particular, those that are synthesized on introns. At the same stage, certain RNA modifications are carried out, such as methylation, the addition of specific groups, etc. correct interaction of RNA with ribosomes, protein factors of translation (see), etc.

Disturbances of process T. can considerably change a metabolism of cells. Defects in enzymes involved in RNA synthesis can cause a decrease in the intensity of T. a large number genes and lead to a significant disruption of the functioning of the cell up to its death.

Genetic defects in the structure of a single T. unit are the cause of a violation of the synthesis of this RNA (and its corresponding protein) and thus can be the basis of a monogenic hereditary pathology (see Hereditary Diseases).

There is a reverse T. - DNA synthesis on an RNA matrix, with Krom, the transfer of information does not occur from DNA to RNA, as in the process of direct T., but in the opposite direction. Reverse T. was first established in RNA-containing oncogenic viruses after an RNA-dependent DNA polymerase, called reverse transcriptase, or reversetase, was found in mature viral particles (see). With the participation of this enzyme in a cell infected with viruses, DNA is synthesized on the RNA matrix, which can later serve as a matrix for the formation of RNA of new viral particles. The viral DNA synthesized by reverse T. can be included in the DNA of the host cell and thereby be the cause of malignant transformation of cells. The return T. in vitro is usually used in researches on genetic engineering (see) for synthesis on matrices of any RNA of structural zones of the corresponding genes.

Bibliography: Ashmarin I.P., Molecular biology, p. 70, L., 1974; 3 en b at sh P. Molecular and cellular biology, trans. from German, vol. 1, p. 135, M., 1982; Kiselev LL RNA-guided DNA synthesis. (Reverse transcription), M., 1978, bibliogr.; Watson J. Molecular biology of the gene, trans. from English, p. 268, M., 1978.

S. A. Limborskaya.

Transcription in biology is a multi-stage process of reading information from DNA, which is a component. Nucleic acid is the carrier of genetic information in the body, so it is important to correctly decipher it and transfer it to other cellular structures for further assembly of peptides.

Definition of "transcription in biology"

Protein synthesis is the main life important process in any cell of the body. Without the creation of peptide molecules, it is impossible to maintain normal life, because these organic compounds are involved in all metabolic processes, are structural components of many tissues and organs, play a signaling and regulatory and protective role in organism.

The process by which protein biosynthesis begins is transcription. Biology briefly divides it into three stages:

Initiation.
Elongation (growth of the RNA chain).
Termination.

Transcription in biology is a whole cascade of step-by-step reactions, as a result of which RNA molecules are synthesized on the DNA template. Moreover, not only information ribonucleic acids are formed in this way, but also transport, ribosomal, small nuclear and others.

Like any biochemical process, transcription depends on many factors. First of all, these are enzymes that differ between prokaryotes and eukaryotes. These specialized proteins help to initiate and carry out transcription reactions accurately, which is important for high-quality protein output.

Transcription of prokaryotes

Since transcription in biology is the synthesis of RNA on a DNA template, the main enzyme in this process is DNA-dependent RNA polymerase. In bacteria, there is only one type of such polymerases for all molecules.

RNA polymerase, according to the principle of complementarity, completes the RNA chain using the template DNA chain. This enzyme has two β-subunits, one α-subunit and one σ-subunit. The first two components perform the function of forming the body of the enzyme, and the remaining two are responsible for retaining the enzyme on the DNA molecule and recognizing the promoter part of the deoxyribonucleic acid, respectively.

By the way, the sigma factor is one of the signs by which this or that gene is recognized. For example, the Latin letter σ with index N means that this RNA polymerase recognizes genes that are turned on when there is a lack of nitrogen in the environment.

Transcription in eukaryotes

Unlike bacteria, transcription is somewhat more complicated in animals and plants. Firstly, in each cell there are not one, but as many as three types of different RNA polymerases. Among them:

RNA polymerase I. It is responsible for the transcription of ribosomal RNA genes (with the exception of the 5S RNA subunits of the ribosome).
RNA polymerase II. Its task is to synthesize normal informational (matrix) ribonucleic acids, which are further involved in translation.
RNA polymerase III. The function of this type of polymerase is to synthesize as well as 5S-ribosomal RNA.

Secondly, for promoter recognition in eukaryotic cells, it is not enough to have only a polymerase. Transcription initiation also involves special peptides called TF proteins. Only with their help can RNA polymerase sit on DNA and begin the synthesis of a ribonucleic acid molecule.

Transcription meaning

The RNA molecule, which is formed on the DNA matrix, subsequently attaches to the ribosomes, where information is read from it and a protein is synthesized. The process of peptide formation is very important for the cell, because normal life activity is impossible without these organic compounds: they are, first of all, the basis for the most important enzymes of all bio chemical reactions.

Transcription in biology is also a source of rRNAs, which are also tRNAs that are involved in the transfer of amino acids during translation to these non-membrane structures. snRNAs (small nuclear nuclei) can also be synthesized, the function of which is to splice all RNA molecules.

Conclusion

Translation and transcription in biology play an extremely important role in the synthesis of protein molecules. These processes are the main component of the central dogma of molecular biology, which states that RNA is synthesized on the DNA matrix, and RNA, in turn, is the basis for the beginning of the formation of protein molecules.

Without transcription, it would be impossible to read the information encoded in deoxyribonucleic acid triplets. This once again proves the importance of the process on biological level. Any cell, be it prokaryotic or eukaryotic, must constantly synthesize new and new protein molecules that are needed at the moment to maintain life. Therefore, transcription in biology is the main stage in the work of each individual cell of the body.

IV. TRANSCRIPTION

Transcription is the first stage in the implementation of genetic information in a cell. During the process, mRNA molecules are formed that serve as a matrix for protein synthesis, as well as transport, ribosomal and other types of RNA molecules that perform structural, adapter and catalytic functions (Fig. 4-26).

Rice. 4-26. Scheme for the implementation of genetic information into phenotypic traits. The implementation of the flow of information in a cell can be represented by the DNA-"RNA-"protein scheme. DNA-"RNA" stands for the biosynthesis of RNA molecules (transcription); RNA-"protein" stands for the biosynthesis of polypeptide chains (translation).

Transcription in eukaryotes occurs in the nucleus. The transcription mechanism is based on the same structural principle of complementary base pairing in the RNA molecule (G ≡ C, A=U and T=A). DNA serves only as a template and does not change during transcription. Ribonucleoside triphosphates (CTP, GTP, ATP, UTP) are substrates and energy sources necessary for the polymerase reaction to proceed, the formation of a 3,5 "phosphodiester bond between ribonucleoside monophosphates.

The synthesis of RNA molecules begins in certain sequences (sites) of DNA, which are called promoters, and ends in the terminating sections (sites of termination). A stretch of DNA bounded by a promoter and a termination site is a transcription unit - transcripton. In eukaryotes, as a rule, one gene is included in the transcripton (Fig. 4-27), in prokaryotes there are several. There is a non-informative zone in each transcripton; it contains specific nucleotide sequences with which regulatory transcription factors interact.

Transcription factors - proteins that interact with certain regulatory sites and speed up or slow down the transcription process. The ratio of informative and non-informative parts in eukaryotic transcriptons averages 1:9 (9:1 in prokaryotes).

Neighboring transcriptons can be separated from each other by non-transcribed DNA regions. The division of DNA into many transcriptons allows for individual reading (transcription) of different genes with different activity.

Only one of the two strands of DNA is transcribed in each transcripton, which is called matrix, the second chain complementary to it is called coding. Synthesis of the RNA chain goes from the 5 "to the 3" end, while the template DNA chain is always antiparallel to the synthesized nucleic acid (Fig. 4-28).

Transcription is not associated with phases of the cell cycle; it can speed up and slow down depending on the need of a cell or organism for a particular protein.

RNA polymerase

RNA biosynthesis is carried out by DNA-dependent RNA polymerases. Three specialized RNA polymerases have been found in eukaryotic nuclei: RNA polymerase I, synthesizing pre-rRNA; RNA polymerase II, responsible for pre-mRNA synthesis; RNA polymerase III, synthesizing pre-tRNA. RNA polymerases are oligomeric enzymes consisting of several subunits - 2α, β, β", σ. The o (sigma) subunit performs a regulatory function, this is one of the transcription initiation factors, RNA polymerases I, II, III, recognizing different promoters, contain structurally different σ subunits.

A. Transcription steps

There are 3 stages in the transcription process: initiation, elongation and termination.

Initiation

The promoter is activated by a large protein - TATA factor, so called because it interacts with a specific promoter nucleotide sequence - TATAAA- (TATA-box)(Figure 4-29).

Attachment of the TATA factor facilitates the interaction of the promoter with RNA polymerase. Initiation factors cause a change in the conformation of RNA polymerase and ensure the unwinding of approximately one turn of the DNA helix, i.e. formed transcription Fork,

Rice. 4-27. The structure of the transcripton.

Rice. 4-28. Transcription of RNA onto DNA template strand. RNA synthesis always occurs in the direction 5 "→ 3".

Rice. 4-29. The structure of the eukaryotic promoter. Promoter elements are specific nucleotide sequences characteristic of any promoter that binds RNA polymerase. The first promoter element, the ATAAA- (TATA-box) sequence, is separated from the transcription start site by approximately 25 base pairs (bp). At a distance of about 40 (sometimes up to 120) b.p. the sequence GGCCAATC- (CAAT-box) is located from it.

in which the template is available to initiate the synthesis of the RNA strand (Fig. 4-30).

After an oligonucleotide of 8-10 nucleotide residues is synthesized, the σ-subunit is separated from the RNA polymerase, and several elongation factors are attached to the enzyme molecule instead.

Elongation

Elongation factors increase the activity of RNA polymerase and facilitate the separation of DNA strands. Synthesis of the RNA molecule proceeds from the 5" to the 3" end of the complementary template DNA strand. At the stage of elongation, in the region of transcription

forks are simultaneously separated by approximately 18 nucleotide pairs of DNA. The growing end of the RNA chain forms a temporary hybrid helix, about 12 base pairs, with the template DNA chain. As the RNA polymerase moves along the template in the direction from the 3" to the 5" end, a divergence occurs in front of it, and behind it, the DNA double helix is restored.

Termination

The unwinding of the DNA double helix in the region of the termination site makes it accessible to the termination factor. RNA synthesis is completed

Rice. 4-30. transcription steps. 1 - attachment of the TATA factor to the promoter. For the promoter to be recognized by RNA polymerase, the transcription complex TATA-factor/TATA-box (promoter) must be formed. The TATA factor remains associated with the TATA box during transcription, which facilitates the use of the promoter by many RNA polymerase molecules; 2 - formation of a transcription fork; 3 - elongation; 4.- termination.

strictly defined sections of the matrix - terminators (sites of termination). Termination factor facilitates separation of the primary transcript (pre-mRNA), complementary to the matrix, and RNA polymerase from the matrix. RNA polymerase can enter the next round of transcription after the attachment of the σ subunit.

B. Covalent modification (processing) of messenger RNA

Primary mRNA transcripts undergo a series of covalent modifications before being used in protein synthesis. These modifications are necessary for the mRNA to function as a template.

Modification 5"-end

Pre-mRNA modifications begin at the elongation stage. When the length of the primary transcript reaches approximately 30 nucleotide residues, capping its 5"-end. Capping is carried out by guanylyl transferase. The enzyme hydrolyzes the macroergic bond in the GTP molecule and attaches the nucleotide diphosphate residue with a 5"-phosphate group to the 5"-end of the synthesized RNA fragment with the formation of a 5", 5"-phosphodiester bond. Subsequent methylation of the guanine residue in composition of GTP with the formation of N 7 -methylguanosine completes the formation of the cap (Fig. 4-31).

Rice. 4-31. Covalent modification of terminal nucleotide residues of the primary mRNA transcript.

The modified 5'-end provides translation initiation, lengthens the lifetime of mRNA, protecting it from the action of 5'-exonucleases in the cytoplasm. Capping is necessary to initiate protein synthesis, since the initiating triplets AUG, GUG are recognized by the ribosome only if a cap is present. The presence of a cap is also necessary for the operation of a complex enzyme system that ensures the removal of introns.

3" end modification

The 3'-end of most transcripts synthesized by RNA polymerase II is also modified, in which a polyA sequence (polyA tail) consisting of 100–200 adenylic acid residues is formed by a special enzyme polyA polymerase.

The signal for the start of polyadenylation is the sequence -AAUAAA- on the growing RNA strand. The enzyme polyA polymerase, exhibiting exonuclease activity, breaks the 3 "-phosphoester bond after the appearance of the specific sequence -AAUAAA- in the RNA chain. To the 3" end at the break point, the polyA polymerase builds up a polyA tail. The presence of a polyA sequence at the 3" end facilitates the release of mRNA from the nucleus and slows down its hydrolysis in the cytoplasm.

Caching and polyadenylation enzymes bind selectively to RNA polymerase II and are inactive in the absence of polymerase.

Splicing of primary mRNA transcripts

With the advent of methods that make it possible to study the primary structure of mRNA molecules in the cytoplasm and the nucleotide sequence of the genomic DNA encoding it, it was found that they are not complementary, and the gene length is several times larger than the "mature" mRNA. Nucleotide sequences present in DNA but not in mature mRNA have been termed non-coding, or introns and the sequences present in mRNA are coding, or exons. Thus, the primary transcript is a nucleic acid (pre-mRNA) strictly complementary to the template, containing both exons and introns. The length of introns varies from 80 to 1000 nucleotides. The sequences of introns are "cut out" from the primary transcript, the ends of the exons are connected to each other. This RNA modification is called "splicing"(from English, to splice- splice). Splicing occurs in the nucleus, and "mature" mRNA enters the cytoplasm.

Eukaryotic genes contain more introns than exons, so very long pre-mRNA molecules (about 5000 nucleotides) after splicing turn into shorter cytoplasmic mRNA molecules (500 to 3000 nucleotides).

The process of "cutting out" of introns proceeds with the participation of small nuclear ribonucleoproteins (snRNPs). The snRNP is composed of small nuclear RNA (snRNA), the nucleotide chain of which is linked to a protein backbone consisting of several protomers. Various snRNPs are involved in splicing (Fig. 4-32).

Nucleotide sequences of nitrons are functionally inactive. But at the 5'- and 3'-ends, they have highly specific sequences - AGGU- and GAGG-, respectively (splicing sites), which ensure their removal from the pre-mRNA molecule. Changing the structure of these sequences affects the splicing process.

At the first stage of the process, snRNPs bind to specific sequences of the primary transcript (splicing sites), and then other snRNPs join them. During the formation of the spliceosome structure, the 3' end of one exon approaches the 5' end of the next exon. The spliceosome catalyzes the cleavage reaction of the 3",5"-phosphodiester bond at the border of the exon with the intron. The intron sequence is removed and the two exons are joined. The formation of a 3",5"-phosphodiester bond between two exons is catalyzed by snRNAs (small nuclear RNAs) that are part of the spliceosome structure. As a result of splicing, “mature” mRNA molecules are formed from primary mRNA transcripts.

Alternative splicing of mPNE primary transcripts

For some genes, alternative splicing and polyadenylation pathways for the same transcript have been described. An exon of one splicing variant may be an intron in an alternative pathway, so mRNA molecules formed as a result of alternative splicing differ in the set of exons. This leads to the formation of different mRNAs and, accordingly, different proteins from one primary transcript. Thus, in the parafollicular cells of the thyroid gland (Fig. 4-33), during the transcription of the calcitonin hormone gene (see Section 11), a primary mRNA transcript is formed, which consists of six exons. Messenger RNA calcitonin is formed by splicing the first four exons (1-4). The last (fourth) exon contains a polyadenylation signal (sequence -AAUAAA-) recognized by polyA polymerase in parafollicular thyroid cells. The same primary transcript in brain cells during another (alternative)

Rice. 4-32. RNA splicing. The splicing process involves various snRNPs that form the spliceosome. snRNPs, interacting with RNA and with each other, fix and orient the reaction groups of the primary transcript. The catalytic function of spliceosomes is due to RNA components; such RNAs are called ribozymes.

Rice. 4-33. Alternative splicing of the calcitonin gene. In thyroid cells, splicing of the primary transcript leads to the formation of calcitonin mRNA, which includes 4 exons and a polyA sequence, which is formed after transcript cleavage in the first region of the polyadenylation signal. In brain cells, mRNA is formed containing: exons 1, 2, 3, 5, 6 and a polyA sequence formed after the second polyadenylation signal.

The splicing pathway is converted into mRNA for a calcitonin-like protein responsible for taste perception. The messenger RNA of this protein consists of the first three exons, which are common with calcitonin mRNA, but additionally includes the fifth and sixth exons, which are not characteristic of calcitonin mRNA. The sixth exon also has a polyadenylation signal -AAUAAA-, recognized by the polyA polymerase enzyme in nervous tissue cells. The choice of one of the pathways (alternative splicing) and one of the possible polyadenylation sites plays an important role in tissue-specific gene expression.

Different splicing variants can lead to the formation of different isoforms of the same protein. For example, the troponin gene consists of 18 exons and codes for numerous isoforms of this muscle protein. Different isoforms of troponin are formed in different tissues at certain stages of their development.

B. Processing of primary transcripts of ribosomal RNA and transfer RNA

The genes encoding most of the structural RNAs are transcribed by RNA polymerases I and III. Nucleic acids - precursors of rRNA and tRNA - undergo cleavage and chemical modification (processing) in the nucleus.

Post-transcriptional modifications of the primary tRNA transcript (tRNA processing)

The primary transcript of tRNA contains about 100 nucleotides, and after processing - 70-90 nucleotide residues. Post-transcriptional modifications of primary tRNA transcripts occur with the participation of RNases (ribonuclease). Thus, the formation of the 3'-end of tRNA is catalyzed by RNase, which is a 3'-exonuclease that "cuts off" one nucleotide until it reaches the sequence -SSA, the same for all tRNAs. For some tRNAs, the formation of the -CCA sequence at the 3 "end (acceptor end) occurs as a result of the sequential addition of these three nucleotides. Pre-tRNA contains only one intron, consisting of 14-16 nucleotides. Removal of the intron and splicing leads to the formation of a structure called "anticodon",- a triplet of nucleotides that ensures the interaction of tRNA with the complementary mRNA codon during protein synthesis (Fig. 4-34).

Post-transcriptional modifications (processing) of the primary rRNA transcript. Ribosome formation

Human cells contain about a hundred copies of the rRNA gene, localized in clusters on five chromosomes. rRNA genes are transcribed by RNA polymerase I to produce identical transcripts. Primary transcripts are about 13,000 nucleotide residues long (45S rRNA). Before leaving the nucleus as part of a ribosomal particle, the 45 S rRNA molecule undergoes processing, resulting in the formation of 28S rRNA (about 5000 nucleotides), 18S rRNA (about 2000 nucleotides) and 5.88 rRNA (about 160 nucleotides), which are components ribosomes (Figure 4-35). The rest of the transcript is degraded in the nucleus.

Rice. 4-34. Pre-tRNA processing. Certain nitrogenous bases of tRNA nucpeotides are methylated during processing by RNA methylase and converted, for example, into 7-methylguanosine and 2-methylguanosine (minor bases). The tRNA molecule also contains other unusual bases - pseudouridine, dihydrouridine, which are also modified during processing.

Rice. 4-35. Formation and exit from the nucleus of ribosome subunits. As a result of processing, three types of rRNA are formed from the 45S rRNA precursor molecule: 18S, which is part of the small subunit of ribosomes, and 28S and 5.8S, which are localized in the large subunit. All three rRNAs are produced in equal amounts since they originate from the same primary transcript. The ribosome large subunit 5S rRNA is transcribed separately from the primary 45S rRNA transcript. Ribosomal RNAs, formed during post-transcriptional modifications, bind to specific proteins, and a ribosome is formed.

Ribosome is a cell organelle involved in protein synthesis. The eukaryotic ribosome (80S) consists of two subunits, large and small: 60S and 40S. Ribosome proteins perform structural, regulatory and catalytic functions.

We encounter the concept of transcription by studying foreign language. It helps us to correctly rewrite and pronounce unknown words. What is meant by this term in natural science? Transcription in biology is a key process in the reaction system of protein biosynthesis. It is he who allows the cell to provide itself with peptides that will perform building, protective, signaling, transport and other functions in it. Only the rewriting of information from the DNA locus to the informational ribonucleic acid molecule launches the protein-synthesizing apparatus of the cell, which provides biochemical translation reactions.

In this article, we will consider the stages of transcription and protein synthesis that occur in various organisms, and also determine the significance of these processes in molecular biology. In addition, we will give a definition of what transcription is. In biology, knowledge on the processes of interest to us can be obtained from its sections such as cytology, molecular biology, and biochemistry.

Features of matrix synthesis reactions

For those who are familiar with the basic types of chemical reactions studied in the course of general chemistry, the processes of matrix synthesis will be completely new. The reason for this is as follows: such reactions occurring in living organisms ensure copying of parent molecules using a special code. It was not discovered immediately, it is better to say that the very idea of \u200b\u200bthe existence of two different languages \u200b\u200bfor storing hereditary information made its way over two centuries: from the end of the 19th to the middle of the 20th. To better imagine what transcription and translation are in biology and why they relate to the reactions of matrix synthesis, let us turn to technical vocabulary for an analogy.

Everything is like in typography

Imagine that we need to print, for example, one hundred thousand copies of a popular newspaper. All the material that enters it is collected on the mother carrier. This first sample is called the matrix. Then it is replicated on printing presses - copies are made. Similar processes take place in a living cell, only DNA and mRNA molecules serve as templates in it, and messenger RNA and protein molecules serve as copies. Let's take a closer look at them and find out that transcription in biology is a reaction of matrix synthesis that occurs in the cell nucleus.

The genetic code is the key to the mystery of protein biosynthesis

In modern molecular biology, no one argues about which substance is the carrier of hereditary properties and stores data on all proteins of the body without exception. Of course it's deoxyribo. nucleic acid. However, it is built from nucleotides, and the proteins, information about the composition of which is stored in it, are represented by amino acid molecules that have no chemical affinity with DNA monomers. In other words, we are dealing with two different languages. In one of them, the words are nucleotides, in the other, amino acids. What will act as a translator who will recode the information received as a result of transcription? Molecular biology believes that this role is performed by the genetic code.

Unique properties of the cellular code

This is what the code is, the table of which is presented below. Cytologists, geneticists, biochemists worked on its creation. In addition, knowledge from cryptography was used in the development of the code. Given its rules, it is possible to establish the primary structure of the synthesized protein, because translation in biology is the process of translating information about the structure of a peptide from the language of nucleotides and RNA into the language of amino acids of a protein molecule.

The idea of coding in living organisms was first voiced by G. A. Gamov. Further scientific developments led to the formulation of its basic rules. First, it was established that the structure of 20 amino acids is encrypted in 61 messenger RNA triplets, which led to the concept of code degeneracy. Next, we found out the composition of nonsense codons that play the role of starting and stopping the process of protein biosynthesis. Then there were statements about its collinearness and universality, which completed the coherent theory of the genetic code.

Where does transcription and translation take place?

In biology, several of its sections that study the structure and biochemical processes in the cell (cytology and molecular biology) determined the localization of matrix synthesis reactions. So, transcription occurs in the nucleus with the participation of the enzyme RNA polymerase. In its karyoplasm, an mRNA molecule is synthesized from free nucleotides according to the principle of complementarity, which writes off information about the structure of the peptide from one structural gene.

Then it exits the cell nucleus through the pores in the nuclear membrane and ends up in the cytoplasm of the cell. Here, the mRNA must combine with several ribosomes to form a polysome, a structure ready to meet transport ribonucleic acid molecules. Their task is to bring amino acids to the site of another reaction of matrix synthesis - translation. Let us consider the mechanisms of both reactions in detail.

Features of the formation of i-RNA molecules

Transcription in biology is the rewriting of information about the structure of a peptide from a structural DNA gene to a ribonucleic acid molecule, which is called informational. As we said earlier, it occurs in the nucleus of the cell. First, the DNA restriction enzyme breaks down hydrogen bonds, connecting the chains of deoxyribonucleic acid, and its helix unwinds. The enzyme RNA polymerase attaches to free polynucleotide regions. It activates the assembly of a copy - an i-RNA molecule, which, in addition to informative sections - exons, also contains empty nucleotide sequences - introns. They are ballast and need to be removed. This process in molecular biology is called processing or maturation. It completes the transcription. Biology briefly explains this as follows: only having lost unnecessary monomers, the nucleic acid will be able to leave the nucleus and be ready for further stages of protein biosynthesis.

Reverse transcription in viruses

Non-cellular life forms are strikingly different from prokaryotic and eukaryotic cells not only in their external and internal structure, but also by matrix synthesis reactions. In the seventies of the last century, science proved the existence of retroviruses - organisms whose genome consists of two RNA chains. Under the action of the enzyme - reversetase - such viral particles copy DNA molecules from ribonucleic acid sections, which are then introduced into the karyotype of the host cell. As you can see, the writing off of hereditary information in this case goes in the opposite direction: from RNA to DNA. This form of encoding and reading is typical, for example, for pathogenic agents that cause different kinds oncological diseases.

Ribosomes and their role in cellular metabolism

Reactions of plastic exchange, which include the biosynthesis of peptides, proceed in the cytoplasm of the cell. To obtain a ready-made protein molecule, it is not enough to copy the nucleotide sequence from a structural gene and transfer it to the cytoplasm. Structures are also needed that will read information and ensure the connection of amino acids into a single chain through peptide bonds. These are ribosomes, the structure and functions of which great attention focuses on molecular biology. We have already found out where transcription occurs - this is the karyoplasm of the nucleus. The place of translation processes is the cellular cytoplasm. It is in it that the channels of the endoplasmic reticulum are located, on which protein-synthesizing organelles, ribosomes, sit in groups. However, their presence does not yet ensure the onset of plastic reactions. We need structures that will deliver protein monomer molecules - amino acids - to the polysome. They are called transport ribonucleic acids. What are they and what is their role in translation?

Amino acid carriers

Small molecules of transport RNA in their spatial configuration have a section consisting of a sequence of nucleotides - an anticodon. For the implementation of translational processes, it is necessary that an initiative complex arise. It should include the template triplet, ribosomes, and the complementary region of the transport molecule. As soon as such a complex is organized, this is a signal to start assembling the protein polymer. Both translation and transcription in biology are assimilation processes, always occurring with the absorption of energy. For their implementation, the cell prepares in advance, accumulating a large number of molecules of adenosine triphosphoric acid.

The synthesis of this energy substance occurs in mitochondria - the most important organelles of all eukaryotic cells without exception. It precedes the onset of matrix synthesis reactions, occupying a place in the presynthetic stage of the cell life cycle and after replication reactions. The splitting of ATP molecules accompanies transcriptional processes and translation reactions, the energy released in this case is used by the cell at all stages of the biosynthesis of organic substances.

Translation stages

At the beginning of the reactions leading to the formation of a polypeptide, 20 types of protein monomers bind to certain transport acid molecules. In parallel, the formation of a polysome occurs in the cell: ribosomes are attached to the matrix at the location of the start codon. The start of biosynthesis begins, and the ribosomes move along the mRNA triplets. Molecules that transport amino acids are suitable for them. If the codon in the polysome is complementary to the anticodon of transport acids, then the amino acid remains in the ribosome, and the resulting polypeptide bond connects it to the amino acids already there. As soon as the protein-synthesizing organelle reaches the stop triplet (usually UAG, UAA or UGA), translation stops. As a result, the ribosome, together with the protein particle, is separated from the mRNA.

How does a peptide get its native form?

The last stage of translation is the process of transition of the primary structure of the protein to the tertiary form, which has the form of a globule. Enzymes remove unnecessary amino acid residues in it, add monosaccharides or lipids, and additionally synthesize carboxyl and phosphate groups. All this occurs in the cavities of the endoplasmic reticulum, where the peptide enters after completion of biosynthesis. Next, the native protein molecule passes into the channels. They penetrate the cytoplasm and ensure that the peptide enters a certain area of the cytoplasm and is then used for the needs of the cell.

In this article, we found out that translation and transcription in biology are the main reactions of matrix synthesis that underlie the preservation and transmission of the organism's hereditary inclinations.

TRANSCRIPTION

Biosynthesis of ribonucleic acid (RNA) molecules on the corresponding sections of deoxyribonucleic acid (DNA) molecules; the first stage in the action of a gene for the realization of genetic information. For the synthesis of RNA, one is used, the so-called. sense strand of a double-stranded DNA molecule. Matrix synthesis RNA (i.e., synthesis using a template, template, in this case, DNA) is carried out by the RNA polymerase enzyme. This enzyme “recognizes” the starting site on DNA (the site of the start of transcription), attaches to it, unwinds the DNA double strand and begins the synthesis of single-stranded RNA. Nucleotides approach the semantic DNA chain, join it according to the principle of correspondence (complementarity), and then the enzyme moving along the DNA crosslinks them into an RNA polynucleotide chain. The growth rate of the RNA chain in Escherichia coli is 40-45 nucleotides per second. The end of transcription is encoded by a special section of DNA. Like other template processes - replication and translation, transcription includes three stages - the beginning of synthesis (initiation), chain extension (elongation) and the end of synthesis (termination). After separation from the matrix, RNA moves from the cell nucleus into the cytoplasm. Messenger RNA (i-RNA), before joining the ribosome and, in turn, becoming a template for protein biosynthesis (translation), undergoes a series of transformations. Thus, the rewriting (lat. "transcription" - rewriting) of the genetic information contained in the DNA nucleotide sequence into the nucleotide sequence of i-RNA takes place. In all organisms, during the transcription of DNA, RNAs of all classes are formed - informational, ribosomal and transport. In 1970, when an enzyme of some tumor-bearing viruses was discovered that performs DNA synthesis on an RNA template, i.e., reverse transcription, the central dogma of molecular biology required clarification.

Encyclopedia Biology. 2012